Voltage follower and related method of regulation

ABSTRACT

A voltage follower includes a follower stage including first and second bipolar junction transistors connected in cascade, and a first current generator connected to the follower stage for biasing the first and second bipolar junction transistors. A cascode stage is connected between the first current generator and the first bipolar junction transistor, and a second current generator is connected between the first bipolar junction transistor and a first voltage reference. The voltage follower dissipates less power when the output current is small.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of electronics, and in particular, to a voltage follower and a corresponding method of regulating the same.

BACKGROUND OF THE INVENTION

[0002] Often in the implementation of complex electronic systems, it is not possible to connect the output of a certain stage to the input of a stage downstream in a signal path. In particular, this is not possible when the downstream stage is to be fed with a small output impedance or with a power greater than that provided by the preceding stage. Usually, in these situations, the upstream stage is coupled to the downstream stage through a voltage follower.

[0003] A voltage follower is a voltage amplifier that outputs a voltage equal to its input voltage independent of the current being absorbed by its electrical load. A typical voltage follower is schematically depicted in FIG. 1, and includes a follower stage formed by the transistors T1 and T2, which are biased by a constant current generator. The output voltage V_(OUT) of the follower stage is a replica of the input voltage V_(rif) and does not depend on the output current I_(OUT) circulating in the transistor T2. The voltage follower is generally formed using BJT technology because of its simplicity, and because of the good tracking between input and output voltages that can be ensured.

[0004] The current generator generates a bias current I such that the transistor T2 may output the desired maximum current I_(OUT,MAX) to the load. Since β_(min) is the minimum gain of the transistor T2, the following relationship must hold:

I=I _(OUT,MAX)/β_(min)  (1)

[0005] The current circulating in the transistor T1 ranges from a minimum value when the transistor T2 outputs the maximum current I_(OUT,MAX), up to a maximum value I when the output current I_(OUT) is null.

[0006] A current almost equal to I circulates in the transistor T1 as long as a relatively small current I_(OUT) circulates in the transistor T2. Therefore, the transistor T1 must be properly dimensioned to have a substantially constant base-emitter gain even when the current circulating in it becomes relatively large.

[0007] The circuit of FIG. 1 is not very efficient when a relatively small current is being supplied to the load. The power provided by the supply P_(s) is:

P _(s) =Vcc·I+Vcc·I _(OUT) =Vcc·[(I _(OUT,MAX)/β_(min))+I_(OUT)]  (2)

[0008] and the power delivered to the load is:

P _(LOAD) =V _(OUT) ·I _(OUT)  (3)

[0009] Therefore, the power dissipation is:

P _(DISS) =P _(S) −P _(LOAD) =Vcc·[(I _(OUT,MAX)/β_(min))+I _(OUT) ]−V _(OUT) ·I _(OUT)  (4)

[0010] The power dissipation of the voltage follower of FIG. 1 becomes relatively large when I_(OUT) is relatively small, as it may be easily inferred from equation (4). Thus, there is a need for a voltage follower capable of delivering to a load a current I_(OUT) that may assume a certain maximum value I_(OUT,MAX), while dissipating a significantly reduced power when I_(OUT) becomes relatively small.

SUMMARY OF THE INVENTION

[0011] In view of the foregoing background, an object of the present invention is to provide a voltage follower that is efficient for driving a load that generally absorbs a relatively small current and only occasionally a relatively large current.

[0012] The voltage follower according to the present invention dissipates less power than the voltage followers of the prior art as long as it delivers a relatively small output current. This is because the follower stage of the voltage follower is biased by a current that is progressively reduced as the output current delivered to the load decreases. According to one embodiment, the voltage follower comprises a follower stage formed by a pair of bipolar junction transistors, electrically in cascade, and commonly biased by a current generator.

[0013] The voltage follower dissipates a relatively reduced power when the output current is small because it comprises a second current generator. The second current generator is connected between the current node of the input transistor of the voltage follower and a common potential node of the stage. The first bias current generator is in the form of a current mirror. A diode connected transistor of the current mirror is coupled through a cascode stage to the current node of the first transistor connected to the second current generator.

[0014] Another aspect of the present invention is directed to a method of regulating a voltage follower driving a load. The voltage follower comprises a follower stage composed of a pair of bipolar junction transistors in cascade, and which are biased by an adjustable current generator. The method comprises progressively increasing or decreasing the bias current provided by the adjustable current generator as a function of a feedback signal representing the current that the output transistor of the pair of transistors delivers to the load.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The different aspects and advantages of the invention will become even more evident through a detailed description of several embodiments and by referring to the attached drawings, wherein:

[0016]FIG. 1 is a schematic diagram of a typical voltage follower according to the prior art;

[0017]FIG. 2 is a schematic diagram of a voltage follower according to the present invention; and

[0018]FIGS. 3 and 4 are schematic diagrams respectively illustrating the voltage follower of FIG. 2 in BJT and BCD technology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Prior art voltage followers, such as the one depicted in FIG. 1, dissipate a non-negligible power when the output current I_(OUT) becomes relatively small. This is so because the bias current of the follower stage is excessively large in this type of operating condition. Such a large bias current is necessary to allow the transistor T2 to provide for the desired maximum current I_(OUT,MAX) to the load.

[0020] All the time the load absorbs a relatively small current I_(OUT). Only a small part of the bias current is absorbed by the base of the output transistor T2 while the remaining part circulates in the transistor T1 dissipating power. This power dissipation is reduced by a method in accordance with the present invention for regulating a voltage follower.

[0021] The follower stage is biased with an adjustable current generator for varying the generated current as a function of the output current I_(OUT). To this end, a feedback signal I_(d) representative of the current I_(OUT) being delivered to the load is used for increasing or decreasing the generated current as the current I_(OUT) increases or decreases, respectively.

[0022] The method of the invention may be easily implemented using a voltage follower as depicted in FIG. 2. The voltage follower in accordance with the present invention has a follower stage composed of a pair of bipolar junction transistors T1, T2 electrically in cascade, and a current generator formed by a current mirror CURRENT_MIRROR. THE current mirror CURRENT_MIRROR has a certain mirror ratio A that biases the follower stage with a current I_(a).

[0023] The characterizing feature of the circuit of the invention is based upon the fact that it includes a cascode stage T3 forming a feedback line, and a second current generator of a constant current I connected between the input transistor of the follower stage and a common potential node of the circuit. Accordingly, the bias current I_(a) that is generated by the current mirror may be varied as a function of the output current I_(OUT) being delivered to the load.

[0024] When the output current I_(OUT) decreases, the base current I_(b) absorbed by the output transistor T2 decreases and thus the current I_(c) circulating in the transistor T1 increases. As I_(c) increases, the feedback current I_(d) that circulates in the diode connected transistor of the current mirror CURRENT_MIRROR decreases, thus reducing the generated biasing current I_(a).

[0025] The constant current I generated by the added second current generator, and the mirror ratio A of the circuit of FIG. 2 may be designed as follows. The adjustable current generator CURRENT_MIRROR must be capable of outputting a maximum bias current I_(a,MAX) sufficient to let the transistor T2 output the desired maximum current I_(OUT,MAX) to the load. Since β_(min) is the minimum gain of the transistor T2

I _(a,MAX) =I _(OUT,MAX)/β_(min)  (5)

[0026] When the transistor T2 outputs the maximum current T_(OUT,MAX) to the load, a negligible current circulates in the transistor T1. In practice, the maximum feedback current I_(d,MAX) is substantially equal to the constant current I generated by the second current generator:

I_(d,MAX)=I  (6)

[0027] The feedback current circulates in the input diode connected transistor of the current mirror CURRENT_MIRROR through a cascode stage T3 made of a transistor biased in a conduction state by a bias voltage V_(BIAS). When the feedback current assumes its maximum value, the follower stage T1, T2 is biased with a current I_(a,MAX) given by:

I _(a,MAX) =A·I _(d,MAX) =A·I  (7)

[0028] According to equations (5) and (7), the constant current I and the mirror ratio A satisfy the following condition:

A·I=I _(OUT,MAX)/β_(min)  (8)

[0029] Through simple considerations it is possible to demonstrate that the current I_(c) circulating in the transistor T1 is always smaller than the constant current I:

I _(c) <I=I _(OUT,MAX)/(A·β _(min))  (9)

[0030] Equation (9) implies that, when the voltage follower of the invention is delivering a relatively small current to the load, the current IC that circulates in the transistor T1 is A times smaller than the current that circulates in the prior art voltage follower of FIG. 1, under the same operating condition. This allows a reduction in the size of the transistor T1, because the transistor T1 is crossed by relatively reduced currents, thus providing for a reduction in the occupied silicon area.

[0031] A second relation between the design parameters A and I for calculating their values may be obtained by comparing the power dissipated by the voltage follower of the invention with the power dissipated by the prior art voltage follower of FIG. 1 that is given by equation (4). Since β is the base-emitter gain of the transistor T2, for the first Kirchhoff's law

I _(a) =T _(b) +I _(c) =I _(OUT) /β+I−I _(d)  (10)

[0032] Combining equations (8) and (10)

I _(a) +I _(d) =I _(OUT) /β+I _(OUT,MAX)/(A·β _(min))  (11)

[0033] The power P*_(s) provided by the power supply of the voltage follower of the invention is:

P* _(s) =Vcc·(I _(a) +I _(d) +I _(OUT))=Vcc·[I _(OUT) /β+I _(OUT,MAX)/(A·β _(min))+I _(OUT])  (12)

[0034] The power output to the load is

P* _(LOAD) =V _(OUT) ·I _(OUT)  (13)

[0035] thus the power dissipation is

P* _(DISS) =P* _(s) −P* _(LOAD) ==Vcc·[I _(OUT) /β+I _(OUT,MAX)/(A·β _(min))+I _(OUT) ]−V _(OUT) ·I _(OUT)  (14)

[0036] For the same current I_(OUT) circulating in the load, the power dissipation of the voltage follower of the invention shown in FIG. 2 may be reduced by increasing the mirror ratio A.

[0037] To make the power dissipation of the voltage follower of the invention smaller than the power dissipation of the prior art voltage follower of FIG. 1 for a certain output current I_(OUT), the mirror ratio A must satisfy the following relation:

Vcc·[I _(OUT) /β+I _(OUT,MAX)/(A−β _(min))+I _(OUT) ]−V _(OUT) ·I _(OUT) <Vcc·[(I _(OUT,MAX)/β_(min))+I _(OUT) ]−V _(OUT) ·I _(OUT)  (15)

[0038] and thus

A>β·I _(OUT,MAX)/(β·I _(OUT,MAX)−β_(min) ·I _(OUT))  (16)

[0039] The voltage follower of the invention is particularly more advantageous than prior art voltage followers when normally a relatively small output current is absorbed by the load, and only occasionally the transistor T2 is required to output a relatively large current. In this case, the current mirror CURRENT_MIRROR may be designed to have a mirror ratio A that satisfies equation (16) as long as the output current I_(OUT) remains below a pre-established level higher than that of the current that is normally delivered to the load.

[0040] The voltage follower of the invention may be conveniently formed either in BJT technology, as shown in FIG. 3, as well as in BCD technology. In the latter case, both the cascode stage and the current mirror are made with MOS transistors, as shown in FIG. 4. 

That which is claimed is:
 1. A method of regulating a voltage follower feeding a load and comprising a follower stage (T1, T2) composed of a pair of bipolar junction transistors first (T1) and second (T2) in cascade, biased by a current generator, comprising the steps of increasing or decreasing progressively the bias current (I_(a)) generated by said current generator in function of a feedback signal (I_(d)) representative of the current (I_(OUT)) that said second transistor (T2) delivers to the load, as the current delivered to the load (I_(OUT)) increases or decreases, respectively.
 2. The method of claim 1, wherein said feedback signal (I_(d)) is obtained as the difference between a pre-established constant current (I) and the current (I_(c)) circulating in said first transistor (T1).
 3. A voltage follower comprising a follower stage (T1, T2) composed of a pair of bipolar junction transistors first (T1) and second (T2) in cascade, biased by a current generator, characterized in that it comprises a second current generator connected between a current terminal of said first transistor (T1) and a common potential node of the circuit; and said first current generator is in the form of a current mirror, a diode-connected transistor of which is coupled through a cascode stage (T3) to the current terminal of said first transistor (T1) connected to said second current generator.
 4. The voltage follower of claim 3, wherein said transistors first (T1) and second (T2) are bipolar junction transistors of opposite type of conductivity.
 5. The voltage follower of claim 3 or 4, wherein said second current generator generates a constant current (I) not smaller than the ratio between the maximum current to be delivered to a load (I_(OUT,MAX)) and the product between the minimum gain value (Pmin) of said second transistor (T2) and the mirror ratio (A) of said current mirror; and said current mirror has a mirror ratio (A) greater than the ratio between the product between the gain (β) of said second transistor (T2) when delivering a pre-established current (I_(OUT)) to the load and the maximum current (I_(OUT,MAX)), and the difference between the said product and the product between said minimum gain (β_(min)) and said pre-established current (I_(OUT)).
 6. The voltage follower according to any of claims from 3 to 5, wherein said cascode stage (T3) is a transistor biased in a conduction state with a bias voltage (V_(BIAS)).
 7. The voltage follower according to any of claims from 3 to 6, wherein the transistors of said current mirror and of said cascode stage are MOS transistors.
 8. The voltage follower according to any of claims from 3 to 6, wherein the transistors of said current mirror and of said cascode stage are bipolar junction transistors (BJT). 